A convicted terrorist banned from Britain for being a risk to national
security has been stopped from being deported by the courts.

The Muslim man, who cannot be named, was found guilty of terrorism in Tunisia and has already been extradited once to Italy, where he was accused of being involved in helping to send Islamists to fight in Iraq and Afghanistan.

Theresa May, the Home Secretary, ordered that he be kept out of Britain because his presence would not be "conducive to the public good for reasons of national security", adding that there was evidence he had been involved in "extremist radicalisation".

However, after he was acquitted in Italy, he returned to Britain and has been allowed to stay by the Court of Appeal while he fights Mrs May's ruling. The court's decision has exposed what experts said was a "loophole" in immigration law which would allow "dangerous" people to stay here.

Experts said the case would have serious implications for the Home Office's ability to exclude terrorists and those suspected of terrorist offences, effectively creating an open border for terrorists while they pursue legal challenges.

Although only a small number of people would be affected, they could pose serious risks to national security.

The Tunisian terrorist is likely to use human rights legislation to argue that it is unsafe for him to be returned to his home country, so he can stay permanently in Britain.

Lord Justice Pill, Lord Justice Rix and Lord Justice Lloyd backed the terrorist's right to be in Britain while he mounts an appeal. Lord Justice Pill commented that denying him access to Britain could lead to "potential injustice". The case puts the judiciary and the Government at loggerheads once again, and comes after the Lord Chief Justice, Lord Judge, warned MPs against using parliamentary privilege to reveal the identities of those who have obtained injunctions to protect their privacy.

Anthony Glees, a professor of security and intelligence at the University of Buckingham, said: "The judiciary are putting the rights of terrorists and terrorist suspects above the rights of ordinary people in this country to go about their daily lives in peace.

"This is a very dangerous precedent if it is allowed to stand. It used to be the case that if the home secretary said someone could not enter this country then that was the end of the story."

The man, known only in legal documents as MK, was convicted of terrorist offences in his absence by a Tunisian military court. He arrived in this country in 2001 and successfully claimed asylum.

The Sunday Telegraph knows the identity of MK but has been prevented from disclosing it by the courts because he is an asylum seeker. The exact details of his criminal conviction in Tunisia are not known.

He lived in Manchester with his wife and daughters. It has been claimed he is a veteran of the Bosnian war, where he fought with the Muslim Mujahideen Brigade.

MK was arrested in a dawn raid by Greater Manchester Police in November, 2007, on a European arrest warrant at the request of an Italian investigating magistrate.

The 53-year-old was held as part of a series of coordinated raids across Europe against an alleged north Italy-based network recruiting fighters for Iraq and Afghanistan. Seventeen were detained in total and a senior Italian politician said at the time that the raids had “decapitated” a “Salafist jihadi” network of terror cells.

Poisons and ignition devices for explosives were seized in a number of northern Italian cities. The suspects were also accused of falsifying identity documents allowing them to travel clandestinely from one European country to another.

Court documents show that MK was accused of “membership of a criminal organisation for the purposes of terrorism”. British police also claimed that the network was involved in forging documents between 2003 and 2005.

He then fought a lengthy legal action, using human rights laws, against extradition, arguing that he could ultimately be sent to Tunisia, where he would be tortured. His action failed and he was sent to Italy in 2008, where he was charged with offences including providing forged documents to jihadis.

In July, he was acquitted of terror-related charges in Italy but convicted of falsely procuring a document. Because of time spent on remand he did not have to serve a further sentence.

Once again his lawyers launched a human rights case to prevent him being deported to Tunisia.

In April last year the Home Office warned MK that his refugee status could be revoked because there was reason to believe he had been involved in “extremist radicalisation and facilitation”, adding that there were “reasonable grounds for regarding him as a danger to the security of the United Kingdom”.

On Aug 7 last year Italian authorities ordered MK to leave the country within five days. He took a train to Switzerland and was arrested at Zurich airport on Aug 25 trying to board a plane to Dublin.

The Swiss decided to send him to Britain — possibly because he had a travel document issued by the Home Office — and he was arrested on the morning of Aug 27 at London City Airport.

When British immigration officers tried to send MK back to Zurich, they were prevented by an injunction. The Home Office tried to overturn the injunction but its case was dismissed by the Court of Appeal.

The challenge hinged on interpretation of the Immigration Act 1971 and other immigration legislation. The court hearing included a debate about the meaning of the word “while” in the phrase “while he is in the United Kingdom” from the 2002 Nationality, Asylum and Immigration Act.

Patrick Mercer, a Conservative MP and security expert, said: “This is particularly worrying in view of the Home Secretary’s efforts to exclude this man. The immigration Acts have been exploited and this loophole needs to be closed.”

A Home Office spokesman said: “We are disappointed by this judgment, but its ramifications are limited. The UK’s exclusions regime will continue to bar people from this country when they seek to subvert our shared values.”